I wrote my own, but I believe there're still errors in BatchedList heading commentary.

1) r.anchor line doesn't have right amount of closing parenthesis
2) choose(item) passes that single item to MyList.new whereas the constructor is awaiting for item list (waiting for
list instead of single item passed).

Moreover, is it better to make internal @current_item which is changed in the callback block, or create new instances
or the controller passing the control through call? I'd guess first thing is better, because state preserving through
backtracking becomes explicit (you have to make the registering call), but most importantly the memory won't become
cluttered with slowly expiring Controller objects.